Who approves a modular or factory-built ADU, HCD or my city?

Both, for different parts of the job. Through HCD's Factory-Built Housing program, an HCD-certified design approval agency approves the plans for the factory-built unit and the unit is inspected in the plant, then it carries an HCD insignia of approval. Your city or county handles zoning, the foundation, the site utilities, and the on-site installation inspections, and it issues the building permit for your project.

What HCD covers

• Factory-built housing is governed by Health and Safety Code section 19960 and following, and it is built to the current California Building Standards Code, the same codes as site-built homes, including the electrical, mechanical, plumbing, energy, and green building codes.
• Every model needs plan approval from an HCD-certified design approval agency, and in-plant inspections are done by HCD, an HCD-approved third party, or a local building department specifically approved by HCD.
• Each unit must bear an HCD insignia of approval. HCD's handbook says units without one may not be installed.

What your city covers

• Plan review of everything not designated as factory-built or not already approved, which often includes the foundation and site work.
• Permit issuance and inspection of the assembly and installation on your lot. HCD's handbook lists underground utilities, on-site electrical, mechanical, and plumbing, and the foundation as local inspection items under Health and Safety Code section 19992.
• Local requirements the law reserves to cities under section 19993, such as zoning, setbacks, snow and wind loads, local fire zones, site development, and architectural standards.
• Any alterations made after installation, which the local building department reviews like work on any other house.

The city generally may not re-review, or charge plan check fees for, factory-built plans already approved by HCD or its approved agency, apart from those local items.

Manufactured homes are a separate path

A HUD-code manufactured home is not factory-built housing. It follows federal standards and Title 25, and the CEC says it is not subject to the Energy Code. Check which label the unit carries before you plan permits.

What it means for your MEP plans

• Find the scope line. Some approvals cover only the factory-built box; others include the foundation and site-installed elements. Everything outside that line needs local plans.
• Site MEP. Typical site plans show the electrical service or feeder and a load calculation for your property, water and sewer connections, any gas piping, and outdoor equipment locations.
• Title 24. Factory-built homes must comply with Title 24, Part 6. Confirm that the manufacturer's energy documentation covers your climate zone and orientation, and who provides the solar PV for a new detached unit.
• Fire zones. If your lot is in a Fire Hazard Severity Zone, local wildfire construction requirements can still apply to the unit.

Read the California ADU requirements guide, the California ADU permit guide, and electrical load calculations.

Sources: hcd.ca.gov, hcd.ca.gov, energy.ca.gov, hcd.ca.gov.